In the recent years, the application of the III-V semiconductor, such as gallium arsenide, as a high speed device in the ULSI (ultra large scalar integral) technology has received more attention than the similar application of the silicon semiconductor, in view of the fact that the III-V semiconductor has a relatively high saturation speed and a relatively low consumption rate of electricity. In the past, the passivation of gallium arsenide (GaAs) in the multilevel interconnection was attained by the thermal oxidation or the anodica. The protective layer so formed is generally composed of the gallium oxide or the arsenic oxide, which is as nonstoichiometrically unstable as the arsenic ion. As a result, the oxide layer of gallium arsenide was seldom used as an insulator.
The silicon dioxide (SiO.sub.2) is devoid of free electrons capable of taking part in the conduction of electrical current in view of the fact that the valence electrons of the silicon dioxide and the adjoining atom form a strong bond which can not be easily severed. As a result, the silicon dioxide has been widely used as the insulation film (band gap=11 eV).
The growth of a silicon dioxide film on the silicon substrate may be achieved by a liquid phase deposition method, which is known as the LPD method for short. For more information on the LPD method, please refer to the publications, such as VLSI FABRICATION PRINCIPLES by S. K. Ghandhi, Wiley, New York, 1983; THIN SOLID FILMS by U. Mackens and U. Merkt, 97, 53 (1982); and C. Chiang, D. B. Fraser and D. R. Denison, IEEE VLSI MULTILEVEL INTERCONNECTION CONFERENCE, 381 (1990). According to the above publications, the LPD method involves a first step in which the silicon dioxide powder is dissolved in an aqueous solution containing 34% of hydrofluosilicic acid so as to form the saturated hydrofluosilicic acid aqueous solution at room temperature. The undissolved silicon dioxide powder is subsequently removed from the saturated hydrofluosilicic acid aqueous solution by means of a filter paper having the diameter of 0.2 .mu.m. The saturated hydrofluosilicic acid aqueous solution is then transformed into a supersaturated solution by adding water, boric acid aqueous solution, or ammonium hydroxide to the saturated solution. The silicon substrate is first cleaned and then immersed in the supersaturated solution in which a silicon dioxide film is formed on the surface of the silicon substrate by deposition. Such reactions as described above may be expressed in terms of the following chemical equations. EQU .DELTA.H+H.sub.2 SiF.sub.6 +2H.sub.2 O.rarw..fwdarw.6HF+SiO.sub.2 .dwnarw.(1) EQU H.sub.3 BO.sub.3 +4HF.rarw..fwdarw.BF.sub.4 +H.sub.3 O.sup.+ +2H.sub.2 O(2) EQU NH.sub.4 OH+HF.rarw..fwdarw.NH.sub.4.sup.+ +F.sup.- +H.sub.2 O(3)
The LPD method has not been used in gallium arsenide substrate. However, the growth of the silicon dioxide film on the gallium arsenide substrate or the III-V semiconductor substrate by the LPD method has become an important technical subject in view of the promising application potential of the gallium arsenide in the ULSI technology.